Magnetic Resonance Imaging (MRI) can generate cross-sectional images in any plane (including oblique planes). Medical MRI most frequently relies on the relaxation properties of excited hydrogen nuclei in water and fat. When the object to be imaged is placed in a powerful, uniform magnetic field the spins of the atomic nuclei with non-integer spin numbers within the tissue all align either parallel to the magnetic field or anti-parallel. The output result of an MRI scan is an MRI contrast image or a series of MRI contrast images.
In order to understand MRI contrast, it is important to have some understanding of the time constants involved in relaxation processes that establish equilibrium following Radio Frequency (RF) excitation. As the high-energy nuclei relax and realign, they emit energy at rates which are recorded to provide information about their environment. The realignment of nuclear spins with the magnetic field is termed longitudinal relaxation and the time (typically about 1 sec) required for a certain percentage of the tissue nuclei to realign is termed “Time 1” or T1. T2-weighted imaging relies upon local dephasing of spins following the application of the transverse energy pulse; the transverse relaxation time (typically<100 ms for tissue) is termed “Time 2” or T2. On the scanner console all available parameters, such as echo time TE, repetition time TR, flip angle α and the application of preparation pulses (and many more), are set to a certain value. Each specific set of parameters generates a particular signal intensity in the resulting images depending on the characteristics of the measured tissue.
Image contrast is then created by using a selection of image acquisition parameters that weights signal by T1, T2 or no relaxation time PD (“proton-density images”). Both T1-weighted and T2-weighted images as well as PD images are acquired for most medical examinations.
A purpose for MR images is to serve as a tool in medical examinations and to aid in establishing a correct diagnosis. For example MR images can be used to find pathological tissue. As is known in the art from e.g. U.S. Pat. No. 7,136,516 during a MR imaging session, the patient is placed inside a strong magnetic field generated by a large magnet. Magnetized protons within the patient, such as hydrogen atoms, align with the magnetic field produced by the magnet. A particular slice of the patient is exposed to radio waves that create an oscillating magnetic field perpendicular to the main magnetic field. The slices can be taken in any plane chosen by the physician or technician performing the imaging session. The protons in the patient's body first absorb the radio waves and then emit the waves by moving out of alignment with the field. As the protons return to their original state (before excitation), diagnostic images based upon the waves emitted by the patient's body are created. The MR image slices are reconstructed to provide an overall picture of the body area of interest. Parts of the body that produce a high signal are displayed as white in an MR image, while those with the lowest signals are displayed as black. Other body parts that have varying signal intensities between high and low are displayed as some shade of gray.
Based on an initial set of MR images the anatomy may be segmented. The segmentation process classifies the pixels or voxels of an image into a certain number of classes that are homogeneous with respect to some characteristic (i.e. intensity, texture, MR parameter values, etc.). For example, in a segmented image of the brain, the material of the brain can be categorized into three major classes: gray matter, white matter, and cerebrospinal fluid. Individual colors can be used to mark regions of each class after the segmentation has been completed. Once the segmented image is generated it can be used for different purposes. For example surgeons can use the segmented images to plan surgical techniques. Another example is quantitative follow-up of the brain tissue volume in case of neuro-degenerative diseases.
Furthermore in the international patent application WO2008/082341, a method and a system for synthetic generation of MR images is disclosed. The method relies on computed values for different parameters of an MR image. In particular the parameters T1, T2 and PD are used.
There is a constant demand to improve the information that can be deduced from an MR image. Hence there is a need for an improved method and system for visualizing MR images.